A mechanism of this type is known from document WO 99/64936, which more generally discloses a method for transmitting mechanical energy pulses from a driving source to an oscillating regulator via a blade spring operating in a buckling manner. More particularly, this method is implemented in particular using an escapement mechanism illustrated in FIG. 1, designed to maintain the oscillations of a regulator, of the sprung balance 10 type, for example, by delivering energy to it received from a driving source, such as a barrel for example, not shown in the drawing, via a blade spring 12, the ends of which are positioned such that it occupies a stable position corresponding to a second mode buckling.
The mechanism includes a plate 14 provided with an impulse-pin 16, mounted on the balance 10. The mechanism also includes a first detent yoke 18, ending with a fork 20 of a traditional type, provided with an inlet horn 20a and an outlet horn 20b and a dart 20c, designed to cooperate with the pin 16 and the plate 14, respectively. The lever ends with a tail 22 and also supports first 24 and second 25 protruding active elements, situated in the plane of the blade spring 12.
The mechanism also includes a second winding yoke 26, comprising a central portion and two symmetrical wings, each supporting, at their end, a key-pin assembly 28 and 29, designed to cooperate with the blade spring 12. The central portion also receives third 30 and fourth 31 active elements, designed to cooperate with first 32 and second 34 escapement wheels.
The two yokes 18 and 26 are mounted free in rotation in reference to each other. However, banking and guide means, which will not be described in detail, connect them, but with play, such that a movement of one yoke causes the movement of the other, but with a certain staggering.
The first 32 and second 34 escapement wheels are arranged on either side and symmetrically in relation to a line passing through the axes of rotation of the balance 10, the yokes 18 and 26 and via the curvature point of the blade spring 12. The wheels 32 and 34 each include a pinion 36 and 38 and mesh with the last wheel 40 of the going train. The wheels 32 and 34 include a particular toothing, the shape of which is adapted to cooperate with the first and second active elements of the second yoke, on one hand to transmit energy to that yoke and, on the other hand, to block the rotation of the wheels, according to operating phases that will be summarized below. For more details, see the document cited in the introduction.
During the main part of an operating cycle, the escapement wheels 32 and 34 can pivot and are not blocked through contact with the third 30 and fourth 31 active elements of the second yoke 26. Thus, in a winding phase, when the balance 10 performs its additional arc, the first escapement wheel 32 turns freely and the second escapement wheel 34 cooperates with the fourth active element 31 of the second yoke 26 to cause it to pivot. The keys-pins 28 and 29 then exert two opposing forces on the blade spring 12, identical and symmetrical in relation to its curvature point. The blade spring 12 then leaves its initial stable state corresponding to a second mode buckling and deforms while winding, without, however, acting on the first yoke 18 at its active elements 24, 25. At this stage, the relative rotational play between the yokes 18 and 26 allows the first yoke 18 to remain immobile.
The balance 10 freely continuing its rotation, the escapement wheels 32 and 34 also continue their movement, until the second wheel 34 locks on the fourth active element 31. The second yoke 26 has continued its pivoting, and the keys-pins 28 and 29 have acted on the blade spring 12, which has continued its winding to a metastable state close to an unstable state corresponding to a fourth mode buckling. The blade spring 12 is then maximally wound. By cooperating with the tail of the first yoke 18, the fourth active element 31 positions the first 24 and second 25 active elements.
During the following step, the balance 10 continuing its oscillation, the pin 16 strikes the inlet horn 20a of the fork 20. The first yoke 18 then acts on the blade spring 12 via the first active element 24. The blade spring 12 then suddenly tilts from its unstable position to a stable state corresponding to a second mode buckling opposite the previous one. This change of state allows the blade spring 12 to act on the keys-pins 28 and 29, which causes the second yoke 26 to pivot, driving the unlocking of the second escapement wheel 34. The second yoke 26 pivots until the third active element 30 encounters one of the teeth of the first escapement wheel 32. During the change of state of the blade spring 12, this also acts on the second active element 25 of the first yoke 18, thereby communicating to the balance 10 the energy accumulated during the winding of the blade spring 12, via the outlet horn 20b. 
During the following alternation, the phases described above are reproduced symmetrically in relation to the plane passing through the axes of rotation of the balance 10, first 18 and second 26 yokes and through the curvature point of the blade spring 12.
Such an escapement mechanism is particularly interesting, in particular for the advantages mentioned in the aforementioned document. More particularly, it makes it possible to obtain an interesting efficiency, by decreasing the stop times of the different elements and the inertias to overcome during operation.
However, it has been observed that adjusting the tension of the blade spring 12 and its position was particularly important to obtain correct operation of the mechanism. In the mechanism disclosed in the aforementioned document, the blade spring 12 is mounted in compression between two settings or using pivot organs. However, adjusting the tension is very delicate with a configuration as proposed in the prior art.
The present invention aims in particular to resolve this problem. It also proposes a particularly advantageous embodiment in its implementation.